The 4% Rule and What It Actually Takes to Retire Early

FIRE (Financial Independence, Retire Early) reduces "how much do I need to retire" to a single number derived from spending, not income — the FIRE & Retirement Calculator computes that target from your monthly expenses and then simulates, month by month, whether your current savings rate actually gets you there and when.

The target: the 4% rule, inverted

The default target is monthlyExpenses × 12 / 0.04 — annual expenses divided by a 4% withdrawal rate, which is the same as saying annual expenses times 25. The logic behind 4% comes from historical market-return studies suggesting a portfolio can sustain that withdrawal rate indefinitely without depleting, adjusted for inflation, across most historical market conditions — not a guarantee, but a widely used planning heuristic. The withdrawal rate is adjustable in the tool, so a more conservative 3.5% (a larger target, more buffer) or a less conservative rate is a one-field change, not a different calculation.

The simulation, not just the target

Rather than only reporting the target number, the tool projects your balance forward month by month: current savings compound at your expected annual return (converted to a monthly rate), your monthly contribution is added each month, and the simulation runs up to 100 years or until the balance crosses the target — whichever comes first. That's what turns "you need $1.2M" into "at your current contribution rate, that's 22 years away," which is the number that actually changes behavior.

What moving the return assumption does

Annual return is the single most sensitive input — a swing from a conservative 5% to an optimistic 8% assumption changes the projected timeline by years, not months, because compounding amplifies small rate differences over long horizons. The tool makes that assumption an explicit, adjustable input rather than hiding it, specifically so the projection's optimism level stays visible.

Building the contribution up

The target and timeline only mean something once there's an actual monthly contribution behind them — the Savings Goal Calculator works the reverse direction (a fixed target date, solving for the monthly amount needed), and the Subscription & Expense Tracker is often where the extra contribution room actually gets found.